In general, holographic recording for recording information in a recording medium through the use of holography is performed by superimposing light that carries image information on reference light within the recording medium and by writing a resultingly generated interference pattern into the recording medium. For reproducing the information recorded, the recording medium is irradiated with reference light such that the image information is reproduced through diffraction derived from the interference pattern.
In recent years, volume holography, or digital volume holography in particular, has been developed and is attracting attention in practical fields for ultra-high density optical recording. Volume holography is a method for writing a three-dimensional interference pattern by making positive use of a recording medium in a direction of its thickness as well, and is characterized in that it is possible to enhance the diffraction efficiency by increasing the thickness of the medium, and a greater recording capacity can be achieved by employing multiplex recording. Digital volume holography is a computer-oriented holographic recording method which uses the same recording medium and recording method as with the volume holography, whereas the image information to be recorded is limited to binary digital patterns. In the digital volume holography, analog image information such as a picture is once digitized and developed into two-dimensional digital pattern information, and then it is recorded as image information. For reproduction, this digital pattern information is read and decoded to restore the original image information for display. Consequently, even if the signal-to-noise ratio (hereinafter referred to as SN ratio) during reproduction is somewhat poor, it is possible reproduce the original information with extremely high fidelity by performing differential detection and/or error correction on the binary data encoded.
Conventional optical information recording/reproducing methods that use holography have had a problem that the reproduced information deteriorates in SN ratio if reproduction-specific reference light is also incident on a photodetector for detecting reproduction light. For that reason, in the conventional optical information recording/reproducing methods, information light and recording-specific reference light are often allowed to be incident on the recording medium with a predetermined angle therebetween at the time of recording, so that reproduction light and reproduction-specific reference light can be spatially separated from each other at the time of reproduction. Consequently, the reproduction light, which occurs at the time of reproduction, travels at a predetermined angle with respect to the reproduction-specific reference light. This allows the reproduction light and the reproduction-specific reference light to be spatially separated from each other.
Nevertheless, when the information light and the recording-specific reference light are allowed to be incident on the recording medium with a predetermined angle therebetween at the time of recording so as to spatially separate the reproduction light and the reproduction-specific reference light from each other at the time of reproduction as described above, there arises a problem that the optical system for recording and reproduction becomes greater in size.
Published Unexamined Japanese Patent Application (KOKAI) Heisei 10-124872 discloses a technique of recording an interference pattern obtained between information light and reference light in an information recording layer in which information is recorded through the use of holography, by irradiating the information recording layer with the information light and the reference light on the same side thereof such that they converge at different positions in the direction of thickness of the information recording layer.
This technique, however, has a problem that a special optical system is required for allowing the information light and the reference light to converge at different positions.
Published Unexamined Japanese Patent Application (KOKAI) Heisei 10-124872 mentioned above also discloses a technique in which a part of the cross section of the beam to be projected onto the recording medium is spatially modulated to form information light while reference light is formed of the other part of the cross section of the beam, and an interference pattern obtained therebetween is recorded in the information recording layer. In this technique, a recording medium is used which has a reflecting surface on a side of the information recording layer opposite to the side to be irradiated with the information light and the reference light, so that an interference pattern obtained between the information light yet to impinge on the reflecting surface and the reference light reflected by the reflecting surface and an interference pattern obtained between the reference light yet to impinge on the reflecting surface and the information light reflected by the reflecting surface are recorded in the information recording layer.
This technique, however, has a problem that the amount of information recordable is reduced because information can be carried by only a part of the cross section of the beam projected onto the recording medium.